Date:   Mon, 14 Jan 2019 14:04:18 -0500
From:   Waiman Long <longman@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Matthew Wilcox <willy@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Kees Cook <keescook@...omium.org>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Davidlohr Bueso <dave@...olabs.net>,
        Miklos Szeredi <miklos@...redi.hu>,
        Daniel Colascione <dancol@...gle.com>,
        Dave Chinner <david@...morbit.com>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH v3 4/4] /proc/stat: Call kstat_irqs_usr() only for active
 IRQs

On 01/11/2019 04:02 PM, Thomas Gleixner wrote:
> On Fri, 11 Jan 2019, Matthew Wilcox wrote:
>> On Fri, Jan 11, 2019 at 08:19:33PM +0100, Thomas Gleixner wrote:
>>> On Fri, 11 Jan 2019, Thomas Gleixner wrote:
>>>> --- a/kernel/irq/internals.h
>>>> +++ b/kernel/irq/internals.h
>>>> @@ -246,6 +246,7 @@ static inline void kstat_incr_irqs_this_
>>>>  {
>>>>  	__this_cpu_inc(*desc->kstat_irqs);
>>>>  	__this_cpu_inc(kstat.irqs_sum);
>>>> +	desc->tot_count++;
>>> There is one issue here. True percpu interrupts, like the timer interrupts
>>> on ARM(64), will access that in parallel. But that's not rocket science to
>>> fix.
>> I was wondering about that from an efficiency point of view.  Since
>> interrupts are generally targetted to a single CPU, there's no cacheline
>> bouncing to speak of, except for interrupts like TLB shootdown on x86.
>> It might make sense for the percpu interrupts to still sum them at read
>> time, and not sum them at interrupt time.
> Yes, for regular interrupts the stats are properly serialized and the patch
> works just fine. For true percpu interrupts we just can avoid the update of
> tot_count and sum them up as before. The special vectors on x86 are
> accounted separately anyway and not part of the problem here. That
> show_all_irqs() code only deals with device interrupts which are backed by
> irq descriptors. TLB/IPI/.... are different beasts and handled low level in
> the architecture code. Though ARM and some others have these interrupts as
> regular Linux interrupt numbers. That's why we need to care.
>
> Updated untested patch below.
>
> Thanks,
>
> 	tglx
>
> 8<-------------
>
>  fs/proc/stat.c          |   28 +++++++++++++++++++++++++---
>  include/linux/irqdesc.h |    3 ++-
>  kernel/irq/chip.c       |   12 ++++++++++--
>  kernel/irq/internals.h  |    8 +++++++-
>  kernel/irq/irqdesc.c    |    7 ++++++-
>  5 files changed, 50 insertions(+), 8 deletions(-)
>
> --- a/fs/proc/stat.c
> +++ b/fs/proc/stat.c
> @@ -79,6 +79,30 @@ static u64 get_iowait_time(int cpu)
>  
>  #endif
>  
> +static void show_irq_gap(struct seq_file *p, int gap)
> +{
> +	static const char zeros[] = " 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0";
> +
> +	while (gap > 0) {
> +		int inc = min_t(int, gap, ARRAY_SIZE(zeros) / 2);
> +
> +		seq_write(p, zeros, 2 * inc);
> +		gap -= inc;
> +	}
> +}
> +
> +static void show_all_irqs(struct seq_file *p)
> +{
> +	int i, next = 0;
> +
> +	for_each_active_irq(i) {
> +		show_irq_gap(p, i - next);
> +		seq_put_decimal_ull(p, " ", kstat_irqs_usr(i));
> +		next = i + 1;
> +	}
> +	show_irq_gap(p, nr_irqs - next);
> +}
> +
>  static int show_stat(struct seq_file *p, void *v)
>  {
>  	int i, j;
> @@ -156,9 +180,7 @@ static int show_stat(struct seq_file *p,
>  	}
>  	seq_put_decimal_ull(p, "intr ", (unsigned long long)sum);
>  
> -	/* sum again ? it could be updated? */
> -	for_each_irq_nr(j)
> -		seq_put_decimal_ull(p, " ", kstat_irqs_usr(j));
> +	show_all_irqs(p);
>  
>  	seq_printf(p,
>  		"\nctxt %llu\n"
> --- a/include/linux/irqdesc.h
> +++ b/include/linux/irqdesc.h
> @@ -65,9 +65,10 @@ struct irq_desc {
>  	unsigned int		core_internal_state__do_not_mess_with_it;
>  	unsigned int		depth;		/* nested irq disables */
>  	unsigned int		wake_depth;	/* nested wake enables */
> +	unsigned int		tot_count;
>  	unsigned int		irq_count;	/* For detecting broken IRQs */
> -	unsigned long		last_unhandled;	/* Aging timer for unhandled count */
>  	unsigned int		irqs_unhandled;
> +	unsigned long		last_unhandled;	/* Aging timer for unhandled count */
>  	atomic_t		threads_handled;
>  	int			threads_handled_last;
>  	raw_spinlock_t		lock;
> --- a/kernel/irq/chip.c
> +++ b/kernel/irq/chip.c
> @@ -855,7 +855,11 @@ void handle_percpu_irq(struct irq_desc *
>  {
>  	struct irq_chip *chip = irq_desc_get_chip(desc);
>  
> -	kstat_incr_irqs_this_cpu(desc);
> +	/*
> +	 * PER CPU interrupts are not serialized. Do not touch
> +	 * desc->tot_count.
> +	 */
> +	__kstat_incr_irqs_this_cpu(desc);
>  
>  	if (chip->irq_ack)
>  		chip->irq_ack(&desc->irq_data);
> @@ -884,7 +888,11 @@ void handle_percpu_devid_irq(struct irq_
>  	unsigned int irq = irq_desc_get_irq(desc);
>  	irqreturn_t res;
>  
> -	kstat_incr_irqs_this_cpu(desc);
> +	/*
> +	 * PER CPU interrupts are not serialized. Do not touch
> +	 * desc->tot_count.
> +	 */
> +	__kstat_incr_irqs_this_cpu(desc);
>  
>  	if (chip->irq_ack)
>  		chip->irq_ack(&desc->irq_data);
> --- a/kernel/irq/internals.h
> +++ b/kernel/irq/internals.h
> @@ -242,12 +242,18 @@ static inline void irq_state_set_masked(
>  
>  #undef __irqd_to_state
>  
> -static inline void kstat_incr_irqs_this_cpu(struct irq_desc *desc)
> +static inline void __kstat_incr_irqs_this_cpu(struct irq_desc *desc)
>  {
>  	__this_cpu_inc(*desc->kstat_irqs);
>  	__this_cpu_inc(kstat.irqs_sum);
>  }
>  
> +static inline void kstat_incr_irqs_this_cpu(struct irq_desc *desc)
> +{
> +	__kstat_incr_irqs_this_cpu(desc);
> +	desc->tot_count++;
> +}
> +
>  static inline int irq_desc_get_node(struct irq_desc *desc)
>  {
>  	return irq_common_data_get_node(&desc->irq_common_data);
> --- a/kernel/irq/irqdesc.c
> +++ b/kernel/irq/irqdesc.c
> @@ -119,6 +119,7 @@ static void desc_set_defaults(unsigned i
>  	desc->depth = 1;
>  	desc->irq_count = 0;
>  	desc->irqs_unhandled = 0;
> +	desc->tot_count = 0;
>  	desc->name = NULL;
>  	desc->owner = owner;
>  	for_each_possible_cpu(cpu)
> @@ -919,11 +920,15 @@ unsigned int kstat_irqs_cpu(unsigned int
>  unsigned int kstat_irqs(unsigned int irq)
>  {
>  	struct irq_desc *desc = irq_to_desc(irq);
> -	int cpu;
>  	unsigned int sum = 0;
> +	int cpu;
>  
>  	if (!desc || !desc->kstat_irqs)
>  		return 0;
> +	if (!irq_settings_is_per_cpu_devid(desc) &&
> +	    !irq_settings_is_per_cpu(desc))
> +	    return desc->tot_count;
> +
>  	for_each_possible_cpu(cpu)
>  		sum += *per_cpu_ptr(desc->kstat_irqs, cpu);
>  	return sum;
>
>
That looks good to me. Thanks for providing a more simple solution.

BTW, if the percpu IRQ is known at allocation time, maybe we should just
not allocate a percpu count for the corresponding descriptor.

Cheers,
Longman

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